halotrichite

{{Short description|Highly hydrated sulfate of aluminium and iron}}

{{Infobox mineral

| name = Halotrichite

| boxwidth =

| boxbgcolor =

| image = Mineraly.sk - halotrichit.jpg

| imagesize = 280px

| alt =

| caption = A sample of halotrichite

| category = Sulfate minerals

| formula = FeAl₂(SO₄)₄·22H₂O

| IMAsymbol = Hth{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}

| molweight =

| strunz = 7.CB.85

| dana =

| system = Monoclinic

| class = Prismatic (2m)
(same H-M symbol)

| symmetry = P2₁/c

| unit cell = a = 20.51, b = 24.29
c = 6.18 [Å]; β = 100.99°; Z = 4

| color = Colorless to white, yellowish, greenish

| colour =

| habit = Acicular to asbestiform clusters, incrustations and efflorescences

| twinning =

| cleavage = Poor on {010}

| fracture = Conchoidal

| tenacity = Brittle

| mohs = 1.5–2

| luster = Vitreous

| streak =

| diaphaneity = Transparent, translucent

| gravity = 1.89

| density =

| polish =

| opticalprop = Biaxial (−)

| refractive = n_α = 1.480 n_β = 1.486 n_γ = 1.490

| birefringence = δ = 0.010

| pleochroism =

| 2V = Measured: 35°

| dispersion =

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| diagnostic =

| solubility = Soluble in water

| impurities =

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| other = Astringent taste

| prop1 =

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| references = [http://rruff.info/doclib/hom/halotrichite.pdf Handbook of Mineralogy][http://www.mindat.org/min-1809.html Halotrichite on Mindat.org][http://webmineral.com/data/Halotrichite.shtml Halotrichite data on Webmineral]

}}

Halotrichite, also known as feather alum, is a highly hydrated sulfate of aluminium and iron. Its chemical formula is {{chem2|FeAl2(SO4)4*22H2O}}. It forms fibrous monoclinic crystals. The crystals are water-soluble.

It is formed by the weathering and decomposition of pyrite commonly near or in volcanic vents. The locations of natural occurrences include: the Atacama Desert, Chile; Dresden in Saxony, Germany; San Juan County, Utah; Iceland; Idrija, Slovenia; and Mont Saint-Hilaire, Canada.

The name is from Latin: halotrichum for salt hair which accurately describes the precipitate/evaporite mineral.

;Gallery

File:Halotrichite Hydrous iron aluminum sulfate Corral, California 3009.jpg|Halotrichite from California

File:Halotrichite-179634.jpg|Halotrichite from the abandoned Golden Queen mine on Soledad Mountain south of Mojave, California